1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine by an output signal from an O.sub.2 sensor which measures the oxygen density of an exhaust gas discharged from the engine.
2. Description of the Related Art
At present, a three-way catalyzer is used to convert three noxious gas components contained in an exhaust gas of an engine into innocuous gas components. Namely, noxious carbon monoxide (CO) and hydrocarbon (HC) are oxidized and nitrogen oxides (NO.sub.X) are deoxidized simultaneously by the three-way catalyzer into carbon dioxide (CO.sub.2), water vapor (H.sub.2 O), and nitrogen (N.sub.2) respectively. It is known that the cleaning capacity of the three-way catalyzer is greatly affected by an air-fuel ratio set for the engine. When the air-fuel ratio is lean, the amount of oxygen (O.sub.2) in the exhaust gas is increased to increase the oxidizing action and reduce the deoxidizing action. On the contrary, when the air-fuel ratio is rich, the oxidizing action may be reduced and the deoxidizing action increased. At a stoichiometric air-fuel ratio, the oxidizing and deoxidizing actions are balanced to enable a most efficient operation of the three-way catalyzer.
To improve the cleaning efficiency of the three-way catalyzer, an air-fuel ratio feedback control system is widely adopted in which an O.sub.2 sensor is used for detecting a residual oxygen density in an exhaust gas, to estimate an air-fuel ratio and to bring the air-fuel ratio close to the stoichiometric air-fuel ratio. In the prior air-fuel ratio feedback control system, the O.sub.2 sensor is arranged in an exhaust system located close to a combustion chamber of an engine, i.e., the sensor is positioned at the gathering point of an exhaust manifold located upstream the three-way catalyzer.
A characteristic of the O.sub.2 sensor is that, when exposed to a low temperature atmosphere, the output of the sensor gradually decreases until eventually the sensor becomes inactive. Namely, when the engine is in an idling state, the temperature of an exhaust gas discharged from the engine drops, and, therefore, the timing of the inversion of an air-fuel ratio signal from the O.sub.2 sensor is gradually displaced from a timing corresponding to the stoichiometric air-fuel ratio, and accordingly, the air-fuel ratio is not maintained at the stoichiometric air-fuel ratio. As a result, the idling operation becomes rough and the quality of emissions in the idling state is deteriorated. (A countermeasure to cope with an error caused in the air-fuel ratio feedback control system when the engine is in an idle state for a long time and the O.sub.2 sensor is cooled, is disclosed in Japanese Patent Publication No. 56-7051.)
Recently, engines tend to be high-powered, and when the engine is operated at a high load, the temperature of the exhaust gas from the engine becomes high, and thus increases the thermal load applied to the O.sub.2 sensor arranged at the exhaust manifold. This causes another problem in that the O.sub.2 sensor is soon damaged.
To reduce the thermal load of the O.sub.2 sensor, it has been proposed to fit the O.sub.2 sensor downstream of the exhaust manifold, for example, at an exhaust pipe.
If the O.sub.2 sensor is provided downstream of the exhaust manifold, such as at an exhaust pipe as mentioned above, the O.sub.2 sensor is cooled not only in an idling state but also in a normal running state (particularly, when a vehicle is running in an urban area or is decelerating) so that the O.sub.2 sensor may be cooled to a temperature at which it becomes inactive. As a result, errors such as an erroneous control of the air-fuel ratio feedback and an erroneous learning in a base air-fuel ratio learning control, will occur more often, to deteriorate the emissions, fuel consumption, and driveability. This will be described later in detail.